Neighborhood designations aren't set by the Postal Service. I live in the South End, part of which has the ZIP Code 02118. According to Mr. ZIP, Boston or Roxbury are acceptable place names for 02118, a good chunk of the South End is in 02116, where Mr. ZIP says the only acceptable place name is Boston. Property descriptions on deeds are a pretty reliable source and usually state the neighborhood of the property. Also Bromley Maps, which are available online and at the Land Court are a pretty good tool for seeing what neighborhood is where. The Newmarket area, according to this very simplified BRA map, appears to be in North Dorchester. I couldn't find an overlay of that map that showed streets, but some of the neighborhood borders appear to be streets.